Astronomers have spotted a new moon orbiting Neptune—the first to be discovered in over a decade—by studying images that were archived all the way back in 2009.

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The new moon, known as S/2004 N1,was discovered in images acquired by the Hubble Space Telescope. Mark Showalter, based at the SETI Institute in Mountain View, California, stumbled across the moon when studying images taken in 2009.

Showalter was actually studying images of Neptune's rings, which are incredibly faint. But instead of using long exposures, he stacked multiple, short exposures on top of each other—and out leapt the moon, clear as day. His team confirmed the finding by studying images acquired back in 2004, too.

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The new moon has a nearly circular orbit, and travels around the planet once every 23 hours. But scientists are now scratching their heads over the finding. The moon is so small—just 20 kilometers across—that the astronomers are surprised it managed to survive the chaotic period when Neptune's other moons formed. As New Scientist explains:

Neptune's biggest moon, Triton, is 2705 kilometres wide and orbits backwards – travelling in the opposite direction to the planet's spin. Its large size and wonky orbit led astronomers to believe that Triton was captured by Neptune's gravity about 4 billion years ago and that it destroyed whatever moons the gas giant originally had as it was settling into its new home.

Apart from its slightly baffling existence, there remains one more question, too: what name will it take? It's currently up for grabs, but Neptune's moons are typically named for minor water deities in Greek mythology—so expect something suitably aquatic. [New Scientist]